An isotropic magnetic recording layer is a magnetic layer having substantially the same coercivities and remanent intensities of magnetization measured in three orthogonal directions, e.g. along X, Y and Z axes. Pinguad U.S. Pat. No. 4,451,535, issued May 29, 1984 describes magnetic recording elements comprising isotropic magnetic recording layers. As shown in that patent, such elements exhibit improved magnetic properties such as information storage capacity, intensity of remanent magnetization and increased signal output at short wavelengths, in comparison to comparable prior art elements.
Pinguad U.S. Pat. No. 4,376,714, issued Mar. 15, 1983, describes a process for preparing acicular particles of magnetic iron oxide. In that process, acicular alpha ferric oxide hydrate particles are prepared by adding a ferrous salt solution to an alkaline hydroxide solution to form an aqueous solution of ferrous hydroxide particles and oxidizing such particles; the ferrous salt solution being submitted to reducing conditions before or after the addition of the alkaline hydroxide solution. That patent discloses that the excess of alkaline hydroxide used can be between 50 and 250 percent. The process described in that patent, including appropriate heat treatments of the alpha ferric oxide hydrate particles, provides acicular iron oxide magnetic particles having an average length shorter than 0.3 micrometer e.g. about 0.15 to 0.2 micrometer and an acicularity between 2 and 8. Such magnetic particles can be used to form isotropic magnetic recording media.
As disclosed hereinafter, a process of the type described in Pinguad U.S. Pat. No. 4,376,714 can be modified by oxidizing the ferrous hydroxide particles in the presence of a specified class of iron complexing agents to provide magnetic particles that are particularly useful in forming isotropic magnetic recording media. Such use of the complexing agents alters the crystalline form and the shape of the ferric oxide hydrate particles to provide a novel precursor for the preparation of the magnetic particles. This will be discusssed in greater detail hereinafter.
There is a continuous endeavor in the magentic recording art to increase the signal to noise ratio of magnetic recording elements. One means used to increase this ratio is to reduce the background noise of magnetic layers. It is known that such noise is related to the size of the particles used for preparing the magnetic layers. Accordingly, it is desirable to provide magnetic particles that are as small as possible in order to obtain magnetic recording elements having an improved signal to noise ratio. One objective of this invention is to provide very small particles having excellent magnetic recording/reproducing properties.